Behavioral slowing is one of the cardinal features of human aging, contributing to the debilitating deterioration of motor functions in senescence. Our principal hypothesis for the past five years of research on this Program Project has been that changes in central dopaminergic pathways constitute a fundamental component of age-associated motoric declines. Converging evidence from our studies and others are providing strong support for this hypothesis. Our experimental plan for the next five years is designed to further our understanding of CNS processes underlying behavioral slowing and analyze therapeutic approaches for intervention. Specifically, our studies focus on the dopamine (DA) neurons in the substantia nigra (SN) and their projections to the caudate nucleus, putamen and globus pallidus of the basal ganglia. The proposed studies will analyze key junctions in the neural circuitry regulating motor functions in the basal ganglia, using behaviorally characterized female rhesus monkeys ranging in age from young adulthood to old age (5-25years+) as a model of human aging. Collectively, the three Projects and three supporting Cores in this Program will critically test the following hypotheses: Hypothesis 1 - That while changes in dopaminergic functions occur throughout the basal ganglia, alterations in neural processing in the SN is a principal component of age-associated motor declines. Hypothesis 2 - That functional changes in the basal ganglia dopaminergic system, including in tyrosine hydroxylase (TH), dopamine transporters (DAT) and DA receptors, are closely associated with age-associated motoric declines. Hypothesis 3 - That anatomical changes in normal aging in the basal ganglia are less predictive than functional changes of age-associated declines in motoric performance. Hypothesis 4 - That local administration of the potent dopaminergic trophic factor GDNF (glial cell line-derived neurotrophic factor) into the SN significantly repairs and restores age-associated declines in SN dopaminergic functions.